26. Fresh-Water Mussel (Anodonta Cygnea)

Description

26. Fresh-Water Mussel (Anodonta Cygnea)

Dissected so as to show the viscera in situ.

The animal is suspended by the apex of its foot and fastened on its left side. The mouth is superior, the anus inferior; the heart to the left hand, and the foot to the right. The right mantle lobe, gills, and body-wall have been removed to show the stomach, coils of intestine, liver, and reproductive glands in situ. The mouth lies between the anterior adductor muscle and the base of the foot. It is fringed by the upper and lower lips, which are prolonged into the corresponding labial tentacles, not visible here. The oesophagus is wide, and leads into the stomach, which has a small cavity on its dorsal wall, and in front of the pylorus a depression, the entrance to a small diverticulum, or caecum, generally, but incorrectly, said to lodge the 'crystalline style.' This organ lies in the stomach, and reaches its full development at the approach of autumn. The liver, or hepato-pancreas, surrounds this portion of the alimentary canal. It looks in this preparation white, owing to the removal of its natural dark colouring matter by the action of the alcohol. Its ducts open into the stomach. A black bristle is passed along the first or descending segment of the intestine which passes straight towards the foot. The first coil curves round towards the dorsal or haemal aspect.

The second coil reverses this direction, and curves concentrically and ventrally by the side of the first and beyond it anteriorly. The third coil passes over the first segment of the intestine on its right side, and ascends towards the dorsal aspect between the first segment and the first 'coil. It then runs along the dorsal surface, perforating the ventricle of the heart, and finally opens by an anus behind the posterior adductor muscle into the supra-branchial chamber. A ridge or 'typhlosole' runs along the posterior wall of the first or descending segment of the intestine. Another commences by a swollen or club-shaped end on the ventral or neural wall of the ascending segment, at the spot where it bends abruptly towards the heart, and is continued as far as the anus, as may be seen here. The ingestion of food depends solely on the currents of water set up by the ciliated epithelium lining the alimentary canal from mouth to anus. The generative gland, in this preparation yellow, is mixed up with the liver, surrounds the coils of the intestine with the exception of a small portion of the second coil, and extends between the ascending and descending segments, and even behind the latter. It must be borne in mind that both liver and genital gland are paired organs.

The sexes are separate in Anodonta, and, like other Lamellibranchiata, it has no accessory organs of generation such as occur in the Snail.

The cut edges of the ventricle of the heart, which surround the intestine, may be seen just behind the spot where the ascending segment of the intestine bends backwards. The lower wall of the ventricle is particularly evident, and there is a well-marked space (ventricular cavity) between it and the intestine.

A portion of the pericardial space is visible as a triangular cavity ventrally to,. i. e. to the right in this preparation of, the ventricle. Still more to the right is the organ of Bojanus or nephridium. The two parts of its cavity may be seen in section. Next to the pericardium is the non-glandular thin-walled duct, the walls of which are more closely apposed than in nature, leaving a mere chink. The glandular portion of the organ with its lamellae extends far forwards; it surrounds the tendon of the posterior retractor pedis muscle, which is seen just in front of the posterior adductor muscle, and reaches to the anterior edge of this muscle and even a little to its ventral surface. Behind the posterior adductor the attachment of the left gill to the mantle is displayed. It corresponds to the junction of the tentaculate and non-tentaculate portions of the mantle which constitute the inferior or inhalent, and the superior or exhalent, siphonal notches respectively.

The muscular portion of the foot is well seen in partial section. In locomotion it swells up and protrudes from the shell into the soft mud of the streams and ponds inhabited by these creatures. The rate of motion is slow, and the animals, as they move along, leave a deep furrow behind them.

The two adductor muscles, as seen in this preparation, are equal in size; hence the name Isomya applied to the great group among Lamellibranchiata to which Anodonta belongs. In others, e.g. the freshwater Dreissena, the anterior adductor is small compared to the posterior, hence Heteromya; and in a third order, the Monomya, to which the Oyster and Scallop belong, the anterior adductor is absent altogether, though it is present in the Oyster when the shell and muscles first develope. The anterior adductor is formed in the mantle region above, and in front of the mouth. The posterior adductor may perhaps correspond to the columellar muscles of the Snail.

The crystalline style occurs among Monomya only in Anomia, but is found in most other Lamellibranchiata, either in a special caecum or in the intestinal tract, as in the Unionacea. It is transparent in most instances. Hazay has recently made some researches on its formation and function in the Unionacea. He finds that from spring to autumn the stomach is full of a gelatinous mass in which the crystalline style is slowly differentiated, a process complete by October. The remaining jelly, apparently superfluous food-material, passes into the first section of the intestine, and by November the stomach is empty of it. In the intestine the jelly becomes a thick, compact hyaline body, which is gradually consumed during the winter months. The style appears to act as a stopper, closing the entrance of the pylorus, but it is itself gradually consumed; any remains serve as the nucleus of a new style. The chemical reactions of both bodies prove their albuminoid nature.

The liver in Lamellibranchiata secretes a diastatic as well as a peptic or tryptic ferment; of the two latter, sometimes apparently only the former, as e. g. in the Oyster and Edible Mussel, or both, as in the Scallop (Pecten Jacobaeus) in which the extract is active in alkaline, neutral and acid solutions alike. The liver contains no calcareous cells, only granular and ferment cells (see pp. 116-17).

The heart consists of a median, thick-walled ventricle, and a thin-walled auricle on either side. The auriculo-ventricular apertures are valved. An aorta arises from both ends of the ventricle; the anterior passes above, the posterior below the intestine. The blood spaces are for the most part lacunar, but vessels are found in the walls of the intestine, labial tentacles, and gills (?). There is a median venous sinus lying between the two nephridia. From it blood passes to the nephridia, thence to the gills, and so to the auricles. The blood is colourless, and has colourless corpuscles. The pericardium surrounds the heart. It communicates with the glandular portion of the nephridium, two apertures at the anterior end leading one into each gland. It is thus directly connected with the exterior.

The renal organs or nephridia (=organs of Bojanus) are paired. They lie ventrally to the pericardium, and are divisible into a duct or non-glandular, and a secreting or glandular portion. The former opens by a pore at the side of the body, and is covered by the inner lamella of the inner gill, where it is attached to the side of the visceral mass. It lies under the pericardium, and opens posteriorly into the glandular portion which underlies it. There is a communication anteriorly between the ducts of opposite sides. Other communications have been stated to exist between the ducts and the organ of Keber. The glandular portion contains numerous lamellae, and is greenish in life. It communicates anteriorly with the pericardium (supra). The outer surface of the ducts has a cylindrical epithelium; the inner has an epithelium, of several layers, the outermost cells rounded and ciliated. A similar ciliated epithelium exists in the glandular portion of the organ. The superficial cells of the latter contain yellow-brown urinary concretions. Guanin not uric acid is stated to be found in them. The relations of the organs are illustrated in Pl. vii. fig. 4. Kollmann has recently described ciliated funnels (nephro-stomata ?) on the lamellae of the glandular portion of the organ.

Their number appears to be great, as many as 200 in an Anodonta specially examined from this point of view. The funnels, however, appear to be blind, and not to lead into any system of canals or into the blood-lacunae of the lamellae into which their blind ends project.

The generative organ of each side opens by a pore in front of the opening of the nephridium. The organs are simple racemose glands, and alike in both sexes. They can only be distinguished by the characters of their products. The testis, however, is whitish in colour; the ovary reddish. In the few hermaphrodite forms the gland may be divided into a distinct male and female part, e.g. Cyclas; or the two elements, male and female, may lie side by side in the same caeca as in Ostrea edulis. In the case of the Oyster the two generative products ripen at different times, and hence self-impregnation does not occur, a rule which obtains in most hermaphrodite animals. Anodonta appears to be occasionally hermaphrodite.

In the Unionacea an external difference of shape between the male and female individual is noticeable, and this difference may be considerable.